Anti-redlining laws in the US, passed in 1977 and strengthened in 1995, forced lenders to give mortgages to people they knew might not be able to afford repayments. People who haven't had a full-time job in years were lent six figure sums.

In order to manage the risk this exposed them too, lenders packaged these sub-prime mortgages up with other ones and traded them as derivatives (collateral debt obligations, or CDOs).

People start to default on mortgages, but because the CDOs are so opaque, no one knows how much liability they, or others, are exposed to. So the banks stop lending to each other and the credit crunch begins.

Now the banks which have overextended themselves – lending far more money than they have in deposits, and relying on being able to borrow cheaply to finance their business model – are in serious trouble. Ultimately, it's a cash flow problem.

Word gets out and Britain witnesses the first run on a bank in over 100 years. Confidence evaporates on both sides of the Atlantic. Share prices plummet, as one bank after another is infected. Investment banks, which do not take deposits and therefore rely most heavily on the ability to borrow, are hardest hit.

Two possible policy responses emerge. One, motivated by the idea of moral hazard, says that banks must be allowed to fail. If government bails them out, they'll behave even more riskily in future. Plus, why should the taxpayers fund a welfare state for bankers? The other school of thought stems from the idea of systemic risk, that allowing banks to collapse would endanger the entire financial system (and, by extension, the capitalist economy).

Fears about systemic risk win out. Governments intervene to try and restore confidence. In the US, the Bush administration attempts to buy up all the bad debt, aiming to get banks lending to each other again.

This what happens when a bubble bursts. For years, the availability of cheap consumer goods from emerging economies like India and China kept down inflation. This meant governments and central banks thought they could flood the market with liquidity (i.e. cheap credit) and get away with it. They couldn't. With too much money chasing too few goods, an asset bubble built up. House prices, in particular, were hugely over-inflated. It got worse after 9/11 when, facing an economic downturn, the US and the UK both pumped even more liquidity into the market. With breathtaking arrogance, politicians claimed to have abolished the economic cycle. In reality, they had simply swapped an immediate and relatively minor readjustment for a much harder landing several years down the line.